Rotary down hole pumps such as progressing cavity pumps have been used in water wells for many years. More recently, they have been found especially well-suited for the pumping of very viscous or thick fluids such as crude oil laden with sand. Rotary down hole pumps are generally driven by a drive string consisting of a rotating rod or tube string that extends down a well bore through a well casing or a production tubing. The drive string has a polished rod at its upper end which extends through the well head to facilitate sealing the well head around the periphery of the drive string. The drive string is suspended from and rotated by a drive head assembly associated with the well head. The pump rotor is mounted to the bottom end of the drive string and the pump stator is connected to the production tubing.
The drive head assembly must support the weight of the drive string, which is quite substantial for deep wells. The drive head assembly must also permit vertical displacement of the drive string to permit proper positioning of the pump rotor in the pump stator. In relatively shallow water wells, only very limited vertical displacement of the drive string is required and this can be accommodated by a threaded rod and nut connection between the drive string and the drive head. An example of this type of connection is disclosed in a catalog of the Peerless Pump Division of F.M.C. Corporation, Bulletin No. B-127 (1952). Deep oil well drive strings may require a substantially greater capacity for vertical displacement to maintain a proper position of the pump rotor due to stretching of the drive string, etc. A drive head assembly described by Kulhanek et al in U.S. Pat. No. 4,372,379 discloses a drive head arrangement having the required capacity for vertical displacement. However, for reliable and safe operation of a rotary down hole pump, the drive head must not only permit vertical displacement but must also permit the reliable transmission of sufficient torque to the drive string to accommodate the pumping of very viscous fluids from deep wells.
The drive head assembly disclosed by Kulhanek et al includes a hollow spindle through which the drive string extends, a frame which rotatably supports the spindle on the well head, and a chain and sprocket drive for rotation of the spindle. The drive string is suspended from the spindle by a standard polished rod clamp which rests in a complementary recess in the top of the spindle. This arrangement permits a transmission of torque from the spindle to the drive string. However, standard polished rod clamps are designed to suspend but not to rotate a drive string, and slippage of the clamp under high torque may result in a loss of the drive string down the well which can cause serious damage to the well casing and the down hole pump.
In U.S. Pat. No. 4,716,961 Makins et al disclose a rotating and supporting drive assembly wherein the weight of the drive string is suspended from the top of a drive spindle by a standard polished rod clamp. The drive string is rotated by a dog clutch arrangement similar to the arrangement described in Kulhanek et al, but the polished rod clamp is positioned at the bottom end of the spindle. Although this construction prevents a loss of the drive string down the well upon slippage of the torque transmitting polished rod clamp, slippage of the clamp at high torque and the resulting damage to the polished rod cannot be prevented.
Thus, it is desirable to provide a drive head which will prevent damage to a polished rod due to slippage of a polished rod clamp during the pumping of very viscous fluids from deep wells.